Shelters for protecting individuals from residual nuclear radiation emanating from radioactive airborne particles (fallout) are typically designed for the purpose of attenuating high-energy gamma radiation. In particular, conventional fallout shelters are usually designed as permanent underground structures to provide shielding and are constructed of high density materials to attenuate radiation emanating from radioactive fallout on the ground and on top of the shelter. Such structures are immovable, expensive to construct, and difficult to maintain.
Permanent fallout shelters also have distinct disadvantages in that they can become traps for their occupants in the event of blast or fire of the structure above them. Occupants can also be trapped by their shelters in areas of high-intensity fallout simply because they do not have suitable shelters available to them elsewhere. Finally, after a period of time, permanent shelters can become contaminated by radioactive contaminants, carried by inadvertent human traffic in and out of the shelter, and thus become unusable.
Also, most of the world's population does not live in anticipated prime target areas in the event of nuclear war, but does live in areas conceivably affected by radioactive fallout from nuclear explosions in the target areas.
Moreover, large numbers of people live in areas that can be significantly affected by radioactive contamination from accidents involving nuclear reactors. Construction and maintenance of permanent fallout shelters composed of high-density materials, protecting populations in areas downwind from a nuclear reactor, has not been considered practical. The effectiveness of large-scale evacuations of populations downwind of nuclear accidents is also in doubt due to panic, loss of effective communications, and abandonment of traffic controls.
Further, case studies of World War II disasters show that in general, civilian populations tend to return to the homes once abandoned, expecting to find shelter and life substance. In summary, a need exists for a portable lightweight, low-cost shelter to provide protection from radioactive airborne dust particles.
It is generally known that a simple hole in the ground can provide significant protection fom gamma radiations emanating from surface contaminants. In particular, an individual who positions himself in the hole is effectively shielded by the surrounding earth from radiations from surface particles. Furthermore, the effective protection provided by a hole in the ground is significantly enhanced if the contaminants can be prevented from settling into the hole, above the hole, or adjacent to the walls of the hole. See Crash Civil Defense Program Planning, Volume I, Appendix F, Page 1-7 (1964), Research Triangle Institute.
It is also generally known that the intensity of radioactivity from nuclear weapon fallout decreases relatively quickly. The intensity falls by a factor of 10 after 7 hours, a factor of 100 after 49 hours, and a factor of 1000 after two weeks. A dose above 50 rem over a short period of time (6 to 7 days) in 90% of the cases is fatal to the person exposed, with death occurring within a few weeks. Death may result from the effects of radiation or from opportunistic, commonly known infectious agents. (A rem or "roetgen-equivalent-man" is a measure of biological damage.)
Populations living largely downwind of targets and outside the central zone directly affected by blast and thermal radiation are expected to receive from 900-3000 rem. Such populations will die or be severely injured unless they can be protected from the effects of short-lived radioactive debris and fallout. Since the estimated maximum toleratable does is 25 rem (cumulative) large fractions of such populations can be saved by trenches and/or holes which provide a protection factor of 100 or better, provided that the accumulation of radioactive fallout inside the trench can be prevented. "It is of interest to mention that a simple one-man foxhole, 3 feet in diameter and 4 feet deep, can provide a protection factor of about 40 if fallout is present up to the edge, but not inside. If an area of 3-4 feet wide around the foxhole is kept free of fallout material, a protection factor of 100 or more is possible." The Effects of Nuclear Weapons, Samuel Glasstone, Revised Edition, 1964, page 473, section 0.140.
Radioactive fallout from a nuclear reactor is generally known to be long-lived. Here, the immediate protection of the population from intensive radiation, inhalation, and ingestion of radioactive particular matter is of paramount importance until rescue missions can be executed. Under peacetime conditions, rescue can be expected to take place shortly after an accident. Even in instances where a nuclear weapon explosion causes a nuclear reactor accident (the worse case possible), if the affected population has 30 to 60 minutes' warning and can take shelter in some sort of portable/temporary fallout shelter, the magnitude of injury can be significantly reduced.
Other references which discuss problems of shetler for populations in the event of significant radioactive particulate fallout include:
National Fallout Shelter Program, Sixteenth Report by the Committtee on Government Operations, May 32, 1962, 87th Congress, 2nd Session, House Report No. 1754. PA1 Maintenance of Civil Defense Shelters in the U.S.S.R., by Yu Yu Nammerer et al., July 1967. PA1 Refer to Publications of the Department of the Navy, Bureau of Yards and Docks. PA1 Fallout Protection for Homes with Basements, Revised, May 1967. C. D. Publication, Item 857-D-1. PA1 Strategy for Survival, Martin and Latham, Page 282, 1963. PA1 Foxhole Shielding of Gamma Radiation, Project 2.3-2, JANGLE, Nevada Proving Grounds, October-November 1951, Armed Forces Special Weapons Project, Washington D.C., WT-393. PA1 The Effects of Nuclear War, Office of Technology Assessment, Congress of the United States, Library of Congress Catalog Card Number 79-600080, 1979. PA1 Last Aid, The Medical Dimensions of Nuclear War, E. Chivian M.D., Susanna Chivian, R. J. Lifton, M.D., J. E. Marck, M.D., page 38-39, 1982.